1. Field of the Invention
The present invention relates to a voltage regulating circuit, and more particularly to a voltage regulating circuit for an uninterruptible power supply (UPS).
2. Description of Related Art
With reference to FIG. 2, a conventional voltage regulating circuit for an uninterruptible power supply (UPS) comprises a transformer (10) and a switch (23). The transformer (10) is connected to AC power inputs L and N and comprises a first coil (11) and a second coil (12) connected in series. The first coil (11) is formed with multiple turns and has an end. The second coil (12) is formed with multiple turns with two ends respectively connected to the AC power inputs L and N. The switch (23) is connected to the transformer (10) and has a normal close node (232), a normal open node (233) and a common node (231). The normal close node (232) is connected to the end of the first coil (11). The normal opened node (233) is connected to the AC power input L through the transformer (10). When the input voltage is at a normal level, the common node (231) of the switch (23) is switched to connect to the normal close node (232). The input voltage passes through the first and second coil (11, 12) of the transformer (10) and the switch (23) as an output voltage. When input voltage is at a low level, the common node (231) of the switch (23) is switched to connect to the normal open node (233). The input voltage passes through the second coil (12) of the transformer (10) and the switch (23) to be transformed into an output voltage. The coupling ratio of the output voltage to the input voltage is the ratio of the sum of turns of the two coils (11)(12) to the number of turns of the second coil (12). Therefore the output voltage is higher than the input voltage in voltage level.
However, the input voltage still passes through the transformer (10) even when the input voltage is at a normal level. When the input voltage passes through the transformer (10), the transformer (10) heats up and may be damaged by constantly heating up.
To overcome the shortcomings, the present invention provides a voltage regulating circuit for an UPS to mitigate or obviate the aforementioned problems.